How to move a Cut Loop and Slide using a measure
Add the Edge Loop and place it as you did before. Without deselecting it, press N to bring up the sidebar and select the Item tab. Here, you can view and edit the exact location of the selected loop cut. Just click on the X, Y or Z values.

An easy method to get an exact value from the left edge is to snap the loop cut onto the left edge of the plane (before left-clicking to apply the loop cut). Then, apply the loop cut and add 6cm to the Y value in the sidebar.